A transformer includes two or more multiturn coils of wire placed in close proximity to cause the magnetic field of one coil to link to the magnetic field of the other coil. The transformer is used to transfer electric energy from one circuit to another circuit using magnetic induction. A current transformer, in particular, includes a primary coil connected in series with a circuit carrying the current to be measured, and a secondary coil across which the current is measured. One type of current transformer, known as a three-phase current transformer, is used in a three-phase circuit and includes three sets of primary and secondary windings. Each secondary winding of the three-phase circuit measures the current passing through its respective primary winding.
The magnetic field generated by the current in a primary coil may be greatly concentrated by providing a core of magnetic material on which the primary and secondary coils are wound. This increases the inductance of the primary and secondary coils, so that a smaller number of turns may be used. A closed core having a continuous magnetic path also ensures that practically all of the magnetic field established by the current in the primary coil will reduce the number of turns of the secondary coil.
Eddy currents are currents induced in the magnetic core by the magnetic fields of the primary and secondary windings. To minimize the energy lost due to these eddy currents, the magnetic core is formed by building it up from laminations stamped from sheet iron or steel. These laminations are, for the most part, insulated from each other by surface oxides and sometimes also by the application of varnish. After forming the laminated core, the closed core is sectioned into two parts so that preformed primary and secondary coils may be placed over different laminated legs of the magnetic core. After the primary and secondary coils are placed over the laminated legs, the two parts of the sectioned core are reattached to one another.
The foregoing technique for assembling current transformers suffers from several drawbacks. One drawback is that the foregoing assembly technique may lead to a loss of magnetic core permeability due to assembly interface air gaps, eddy currents, and opposing material grain directions and stresses in stamped core laminations. These deficiencies, in turn, produce relatively large variabilities. Another drawback is that core laminations in the above assembly technique use an involved assembly process requiring stamping, orienting, stacking and fastening of the core laminations. Yet another drawback is that the assembly technique results in inefficient magnetic flux paths, thereby increasing the required size of the current transformers. A further drawback is that the core lamination material used in the assembly technique is relatively thick because the laminations are stamped, oriented, stacked, riveted and in general handled as individual components. The thicker lamination material, in turn, increases eddy currents and bending stresses, resulting in a decrease in magnetic core permeability. Another drawback is that stamping methods used in the assembly technique produce a high percentage of scrap, which results in a waste of expensive magnetic iron material. In addition, these stamping methods produce stresses in the magnetic iron material which degrade magnetic core permeability.
A need therefore exists for a method for assembling a current transformer which overcomes the above-noted drawbacks associated with existing assembly techniques.